In this paper, we report a novel method of fabricating patterned magnetic metal alloy CoCrPt nanodots via selective ion etching using a self-organized array of spherical domains of an ordered block copolymer (PS-b-PMMA) film as a template. Reactive ion etching and Ar⁺ ion milling were controlled accurately to avoid excess etching and pattern loss in the etching processes. The nanodot arrays had a dot density as high as approximately 10¹¹ dots cm⁻², and the magnetic hysteresis loops showed that the coercivity of the isolated CoCrPt dots made using the self-assembled block copolymer was considerably higher than that of the continuous magnetic film. The present method is applicable to other magnetic metals.